The csl:value-of instruction appends generated text to ContentMap item generated by parent csl:template.
<csl:value-of select="xpath-expr" max-length="50"/>
The required select attribute is an XPath expression. This expression is evaluated and result is then converted to string.
The csl:value-of works like xsl:value-of (described in "XSL Transformations 7.6.1 Generating Text with csl:value-of")
The csl:collect-text must be used wherever possible because it is more effective then csl:value-of.